For decades already the NMR (=Nuclear Magnetic Resonance) principle has been employed as a chemical method of analysis. The procedure is based on the behaviour of certain atoms as if they were tiny magnets. These "magnets" may be controlled like common magnets with the aid of an external magnetic field. If a sample containing such atoms (protons) is placed in a strong external magnetic field, the majority of the magnets constituted by protons will orient themselves parallel to the field. The protons are then at their lowest energy level, and therefore the internal energy of the sample is at its minimum. It is however possible to increase the energy level of the sample by irradiating it with electromagnetic radiation, which excites the protons to a higher energy level. Protons are however unable to take up energy other than in quanta of a given size. The quantum size depends on the external magnetic field: EQU E=V Bo
V=constant PA1 Bo=external magnetic field. PA1 h=constant PA1 f=frequency PA1 E=quantum energy. PA1 k=v/h=constant PA1 .rho.=proton density PA1 T.sub.2 =SPIN-SPIN relaxation time PA1 T.sub.1 =SPIN-AMBIENCE relaxation time.
To this quantum corresponds an electromagnetic radiation having the frequency EQU f=E/h
In other words, the exciting frequency is directly proportional to the strength of the magnetic field EQU f=k Bo
Likewise, when returning to a lower energy level, protons emit electromagnetic radiation with a frequency directly proportional to the external magnetic field. Therefore if the sample is placed as it is in a magnetic field of which the strength varies in a known manner dependent on location, one is thereby enabled to excite certain protons or to receive the signals emitted by protons located at a given spot (in a given field strength). Hereon are based various NMR image-forming methods by the aid of which the proton density in various parts of the body is mapped.
The signal obtained from the sample also contains information on the tissue from which the signal originates. An NMR signal S departing from a biological tissue has the form EQU S=.rho.T.sub.2 /T.sub.1
T.sub.1 contains information on the ambience wherein the protons emitting the signal reside. The frequency of the NMR signal is also dependent on the mode of binding of the proton to the surrounding atoms (so-called Chemical Shift). It is possible with the aid of the Chemical Shift, for instance, to distinguish between unbound inorganic phosphorus and e.g. phosphorus bound to ATP. (ATP=Adenosine triphosphate--the energy stores of the biological cell. As the cell consumes energy, ATP divides into ADP and free phosphorus.)
It is possible, by determining in a tissue the average proportions of bound and free phosphorus, to conclude which is the situation of the cells nutritional supply. It is thus possible to distinguish for instance between infarcted tissue and healthy tissue in the cardiac muscle, or to monitor the reactions of the organism to a renal transplant.
But NMR mapping apparatus known in the art, whereof types have been developed e.g. in Great Britain and in the U.S.A., is expensive and implies prolonged image-forming times (several tens of minutes). It is possible in such procedures to produce cross sectional images of the human body just like in computer x-ray tomography. Clinically, however, image-forming is not indispensable; it would suffice if one were able to study a given part of the body, for example the liver, kidney, etc. With this state of prior art as starting point, the object of the invention is an NMR diagnosis apparatus comprising members for producing a magnetic field covering the object of study, a transmitter for transmitting high-frequency electromagnetic radiation to strike the object of study, a receiver for reception of the so-called NMR signal emitted by the irradiated object, and means for processing and analysing the signal.
An apparatus of this kind is known e.g. through the U.S. Pat. No. 3,789,832. One attempts with this apparatus of prior art, by means of NMR image-forming, to achieve a mapping of the whole body in order to detect a malignant growth on the basis of the differences between malignant cancer growth and normal tissue, which could be seen in the changes of the relaxation times in the NMR signal. In malignant tissues, the above-mentioned relaxation time T.sub.1 was substantially shorter than in equivalent normal tissues.
The most expensive components in NMR equipment are the blocks forming the magnetic field. The requisite field strength is between 0.1 and 0.2 T, which is equivalent to a proton frequency of 4 to 8 MHz. It is therefore possible most efficiently to reduce the price of the apparatus by simplifying the magnetic field forming.